From the viewpoint of energy problems and environmental problems, renewable energy is rapidly increasing. Particularly, in EU, there exist countries such as Sweden where the renewable energy makes up 50% of the energy (natural energy).
Since power (electrical energy) consumption always changes depending on human's daily activities, demand and supply balance deteriorates. When atomic energy that is difficult to adjust the output is combined with irregular natural energy to compensate for power consumption, it is feared that generation of surplus power will increase because of a failure in matching to the power consumption. If renewable energy increases in the future, there will be a limit to output adjustment by thermal energy.
Although the surplus power may be transmitted to regions with a short supply of electricity, transmission poses problems of energy loss, cost, and the like. Current measures against these problems are water pumping for pumped storage hydroelectricity and storage of electricity using a storage battery. However, land appropriate for construction of a dam capable of generating power by pumped storage is limited, and the storage battery is expensive.
As described above, electric energy has problems concerning storage and transmission of electricity. On the other hand, an artificial photosynthesis system has been proposed as renewable energy that can be stored and transported.
Artificial photosynthesis is a technology of generating a chemical fuel (chemical energy) by light energy, like plants. The plants use a system called Z scheme that excites light energy in two stages. Using solar energy, the plants oxidize water and obtain electrons, and deoxidize carbon dioxide and synthesize cellulose or saccharides. In the artificial photosynthesis, the photosynthesis reaction of plants is enabled by an artificial photochemical reaction.
In Jpn. Pat. Appln. KOKAI Publication No. 2011-094194, a carbon dioxide (CO2) reducing catalyst is formed on the surface of a photocatalyst. This CO2 reducing catalyst is connected to another photocatalyst by an electric wire. The other photocatalyst obtains a potential by light energy. The CO2 reducing catalyst obtains a reduction potential from the other photocatalyst by the electric wire, thereby reducing CO2 and producing formic acid. Thus, Jpn. Pat. Appln. KOKAI Publication No. 2011-094194 uses two-stage excitation in order to obtain a potential necessary to reduce CO2 by the photocatalyst by using visible light. However, the efficiency of conversion from the sunlight to the chemical energy is as very low as 0.04%. This is so because the energy efficiency of the photocatalyst which is excited by visible light is low.
In Jpn. Pat. Appln. KOKAI Publication No. 10-290017, an arrangement is considered in which a silicon solar cell is used to obtain a reaction potential, and a reaction is caused by providing a catalyst on each of the surfaces of the silicon solar cell. In S. Y. Reece, et. al., Science. vol. 334, pp. 645 (2011), a multilayered structure of silicon solar cells is used to obtain a reaction potential. An electrolytic reaction of water (H2O) is caused by providing a catalyst on each of the surfaces of the structure. In these related arts, the conversion efficiency from sunlight energy to chemical energy is as high as 4.7%.
In these artificial photosynthesis systems, however, the conversion efficiency cannot exceed the sunlight energy conversion efficiency of a solar cell. In other words, since the electromotive force is obtained by charge separation of the solar cell, it is impossible to exceed the sunlight energy conversion efficiency of the solar cell.
For this reason, these artificial photosynthesis systems effectively function in a case with surplus power but not in a case with power shortage. That is, in a case with power shortage, the efficiency of directly converting sunlight energy into electric energy using a solar cell than the efficiency of converting sunlight energy into chemical energy by these artificial photosynthesis systems and then converting the chemical energy into electric energy. Hence, when power supply is short, generating electric energy using a solar cell is more efficient than generating chemical energy using an artificial photosynthesis system.
To solve both the problem in the case with surplus power and that in the case of power shortage, both the artificial photosynthesis system and the solar cell may be installed. However, when the artificial photosynthesis system and the solar cell are installed, the installation area needs to be at least twice larger.
When surplus power exists, and sunlight energy is small, a method of converting electric energy into chemical energy using a normal electrolytic system using a power supply is usable in addition to the artificial photosynthesis system. However, there are problems that the solar cell and the electrolytic system need to be connected, and the device needs to be operated following unstable natural energy.
There is no integrated device that appropriately operates as an artificial photosynthesis system, a solar cell, and an electrolytic system so as to raise the energy conversion efficiency in accordance with conditions such as the presence/absence of surplus power and the presence/absence of sunlight energy.